1. Field of the Invention
The present invention relates to a heat transfer device for cooling a fluid, the device having an air-cooled portion and a contiguous evaporatively cooled portion and to a refrigerating package utilizing such a heat transfer device as a condenser and liquid refrigerant subcooler.
2. Physical Principles and Prior Art
Fluid coolers utilizing air alone as the coolant are well known. Such fluid coolers are known as air-cooled fluid coolers. Fluid coolers utilizing an air stream combined with evaporation of water on the heat transfer surface are also well known. Such fluid coolers are called evaporatively cooled fluid coolers. Evaporative coolers generally have a water reservoir or sump positioned directly beneath a sprayed coil or similar heat transfer surface through which the fluid to be cooled is passed. A pump draws water from the sump and circulates it over the coil through spray nozzles in excess of the evaporation rate while air is blown by fans over the wetted coil surfaces. The excess water falls off the wetted coil surfaces into the sump for recirculation. The evaporation of the water from the external coil surfaces cools the fluid flowing inside. A float in the sump senses the water level and controls the flow of make-up water to replace that lost by evaporation on the wetted coil surfaces. It is generally considered desirable to bleed off a small amount of water from the sump to the drain to prevent accumulation of excess concentrations of dissolved salts which could precipitate on the coil or in the sump causing operational problems.
Evaporation of water from the wetted surfaces of the coil provides a powerful cooling effect because the sink temperature is the wet bulb temperature of the ambient air.
The sink temperature of a coolant is the lowest temperature to which a substance can theoretically be cooled by heat transfer to that coolant. Dry bulb temperature of air is the temperature of air measured by an ordinary thermometer. Wet bulb temperature is the temperature of air measured by the same thermometer used in dry bulb measurements, except the bulb is enclosed within a porous wetted glove or sock. The flow of air over the wetted sock cools the bulb to a temperature lower than the dry bulb temperature by a number of degrees called the wet bulb depression. The wet bulb depression varies with the relative humidity. When the relative humidity is near 100 percent, as in tropical rain forests, the wet bulb depression is small. When the relative humidity is low, as in the desert, the wet bulb depression is large. There are published charts which show the wet bulb temperature and wet bulb depression for each air temperature and relative humidity. These charts are called psychrometric charts.
Since the sink temperature for evaporative coolers is the wet bulb temperature, such coolers have the capability of cooling fluids to lower temperatures, in some cases much lower, than air-cooled coolers.
When an evaporative cooler is used in a refrigeration system as condenser it is called an evaporative condenser. Evaporative condensers are typically much smaller than air-cooled condensers having the same capacity.
Systems having evaporative condensers are frequently employed in an air-cooled mode during cold weather. In the air-cooled mode the recirculated water supply is stopped and cold ambient air alone is circulated over the generally bare (unfinned) evaporative condenser tubes to provide the thermal sink for the heat required to be rejected at the condenser. Because evaporative condensers usually have relatively small heat transfer surfaces for their design heat load, the water circulation generally cannot be stopped until the air temperature is near freezing. This complicates the operation and can lead to freeze-up of the condenser.
It has been common practice to provide a separate subcooling coil submerged in the sump of the evaporative condenser. In many systems the subcooling coil is connected to receive the liquid flow from a liquid receiver. In other systems, especially those having limited refrigerant charge or floating receivers the sub-cooling coil is connected directly to the condenser outlet.
When air-cooled fluid coolers are employed in refrigerating systems as condensers they are known as air-cooled condensers. Though air-cooled condensers have the advantage of operating dry and thereby avoiding the corrosion associated with the use of water for cooling, they exhibit the important limiting disadvantage that the condensing temperature rises degree for degree with the ambient dry bulb temperature. During weather when air temperatures exceed 95.degree. F. air-cooled condensing temperatures can exceed 125.degree. F. This condensing temperature may be too high for system employing refrigerants which get very hot on compression, such as ammonia or HCFC-22 (monochloro difluoromethane). To cope with these high condensing temperatures, users of air-cooled condensing equipment have sometimes provided water sprays for supplementary cooling.
In one such arrangement water is sprayed into the entering airstreams of the air-cooled condenser to provide adiabatic cooling of the airstream before it enters the condenser. This arrangement does not allow water to evaporate directly on the condenser heat transfer surface. As a result the life of the condenser surface is generally not adversely affected.
In another arrangement water is sprayed directly onto the heat transfer surfaces of the air-cooled condenser. In this arrangement the life of the heat transfer surface can be reduced by deposition of minerals onto the tubes and fins of the condenser surface and by corrosion of the heat transfer surface.
Air-cooled condensers are also frequently provided with subcooling coils. These sub-cooling coils are frequently made integral with the condenser coil and are positioned to be subject to the relatively cool condenser inlet airstream. The subcooling coils in some system designs employ a "floating" liquid receiver. In a floating receiver design the sub-cooling coil is connected directly to the outlet of the condenser and the receiver is connected by a single pipe to the main liquid line connecting the sub-cooler outlet to the evaporator. In other designs a flow-through receiver is employed. In this design the receiver has an inlet and an outlet connection and is connected to receive the full flow of condensed liquid from the condenser outlet. The sub-cooling coil is connected in the liquid line connecting the receiver outlet with the evaporator.
As described above, the condensing temperature of air-cooled systems rises degree for degree with the outside ambient temperature. Conversely, as the ambient temperature falls, the condensing temperature also falls degree for degree with the ambient temperature. Though, thermodynamically, this would seem to be a totally desirable characteristic, paradoxically, many systems fail to refrigerate properly when the ambient becomes very cold and the condensing temperature and corresponding liquid pressure becomes very low. Pressure controls to establish minimum condensing temperatures or minimum liquid pressures have been provided to better enable these systems to operate year-round. These controls are frequently called "winter controls" because they allow the system to operate correctly during cold weather. Still other systems have been designed to operate without any winter controls. These systems have been designed to operate correctly, even at sharply reduced condensing temperatures and correspondingly low liquid pressures. In other words, their condensing temperatures have been allowed to float downwards as well as upwards. These systems are called "floating head pressure" systems. Systems with "floating" receivers, i.e. those receivers not directly in the flow stream, can be of either the winter controlled or the "floating head pressure" design.
Both air-cooled and evaporative coolers have been employed to cool recirculating liquid such as glycol/water solutions for circulation through heat exchangers such as water-cooled condensers to provide cooling therefor.